In many high temperature gas filtering processes, where particulate material is removed from a hot gaseous stream, such as in coal conversion systems and cogeneration systems, ceramic, tubular filter elements are used which are supported in apertures in a tube sheet. The ceramic, tubular filter elements are candle-shaped bodies that are formed from a ceramic material and have porous walls that enable the passage of gases through the walls to an interior chamber from which the filtered gases are discharged through an open top of the filter element, while the particulates are collected on the outer surface of the porous walls: At predetermined intervals, when the passage of the gases through the porous walls is retarded due to buildup of particulates on the outer surface of the porous walls, a blowback system is activated which directs a pressurized flow of a gas into the interior chamber through the open top of the filter element and through the porous walls, in a direction opposite to that of the hot gases being filtered, to dislodge the collected particulates from the outer surface of the filter element and clean the filter for further use.
Such filtering systems are known in the art, but a problem exists in providing an adequate seal between the ceramic, tubular which the filter elements are supported. The seal is difficult because it must be effected between rigid porous ceramic materials that comprise the filter elements and metallic mountings or tube sheets. These seals are subjected to diverse forces and must provide a seal under various stresses. For example, such a seal must evidence an ability to maintain sealing loads at elevated temperatures; have the ability to accommodate considerable differential motion of the components due to mechanical loads as well as differential thermal expansion of the components; have the ability to accommodate vibrations in the system; have the ability to maintain sealing force under conditions of changing gasketing properties and deformations in the easily after prolonged exposure to high temperatures and possibly corrosive gaseous atmospheres.
In order to prevent passage of unfiltered gases or particulates between the metallic tube sheet and the ceramic, tubular filter elements supported thereby, a gasketing material is disposed therebetween. The gasketing material cushions the contact between the ceramic, tubular filter element and the metallic tube sheet and provides a barrier to the passage of gases and/or particulates. The use of metallic springs to bias the ceramic, tubular filter element against the gasket, and thus against the tube sheet has been proposed. In a copending application, Ser. No. 817,704 filed Jan. 10, 1986, entitled "Apparatus and Process for Filtering High Temperature Gas Streams" filed in the names of Gordon Isrealson, J. Schwab, and the present inventors, assigned to the assignee of the present invention, for example, the use of a wave spring is proposed to bias the top portion of a filter element towards a gasket in a tube sheet. These types of seals are ordinarily effective using metallic springs of appropriate characteristics. Applications that require high temperature sealing, however, may not be serviced by simple metallic springs because the metals display a sharp drop in modulus of elasticity and become soft and yielding at temperatures in the range of 700.degree.-900.degree. C. When such properties are lost, the spring biasing action becomes ineffective.
Current technology usually employs one of three methods for effective sealing under such high temperature conditions.
The assembly can be spring loaded using external studs, rods and springs. In this type of system, the casing is spring loaded against the ceramic interior which operates at high temperature. The springs and casing are insulated and are not directly exposed to thB high temperature of the internal ceramic component. In this application, the externally applied load also serves the additional function of placing the ceramic body in a state of compression that greatly reduces mechanical failures due to internal pressure which can cause delamination of the body in tension. A commercial implementation of this technique, is available from GTE Sylvania which uses an externally loaded system in its line of recuperative heat exchangers. A major drawback of this mounting system is that it greatly limits the size and geometry of the assembly and also limits a unit to low pressure applications since the metallic casing is unsealed and incapable of supporting large pressure loads without significant leaking.
In recent developments, the very large surface area to volume ratio of the cross flow configuration has been exploited in its use as a high temperature, high pressure (HTHP) filter for application in advanced coal conversion technologies such as pressurized fluid bed combustion (PFBC) and pressurized fluid bed gasification of coal. These applications are described in U.S. Pat. No. 4,343,631, and in the above-identified cross-reference related application of the present inventors, both of which are assigned to the assignee of the present invention, and both of which are incorporated by reference herein. Early methods for sealing a ceramic cross flow filter, as illustrated in FIG. 7 of U.S. Pat. No. 4,343,631 relied on a metallic plate being placed across the back (sealed) end of the filter and the use of threaded studs to subsequently pull the filter element tightly up to a gasketed manifold thereby forming a dust tight seal. Difficulties have been experienced with this method of mounting because of differential thermal expansion of the studs relative to the ceramic. This has caused frequent failure of the seal because of a gradual loosening of the assembly upon thermal cycling.
The more recent embodiment of this general concept has been disclosed in said above-identified cross-reference related application In this system, methods have been provided for the incorporation of a flange on the front sealing face of the element which has been mounted in the "face down" orientation so as to minimize the gravity loads on the seal of the unit. In this system, the problem of loosening are mitigated, but not eliminated, by keeping the clamping stud length at a minimum, and thereby minimizing total differential growth of the clamping member. Problems do exist in this sealing method since it requires a bolting force to be applied to the ceramic flange directly, or for the sealing force to be transmitted through the compression of fibrous ceramic gasketing material. Loading the retaining flange directly onto the ceramic flange is quite difficult because the ceramic member is so brittle that is likely to crack and cause failure. When a layer of ceramic gasketing is used to cushion and transmit the sealing force to the flange, there is the likelihood that the gasketing material will lose its resiliency after long term exposure to high temperature and repeated thermal and mechanical cycling. When this has occurred, the seal is compromised and rapid deterioration of the closure ensues.
Applications where ceramic, tubular filter elements are to be sealed to metallic tube sheets, as presently known, are illustrated in FIGS. 1 and 2 of the present application. In these applications, a plurality of flanged ceramic, tubular filter elements are required to be sealed in a large metallic tube sheet. As illustrated in FIG. 1, a filter unit A has a ceramic, tubular filter element 3 which is comprised of a hollow cylindrical member that has a closed bottom 5, side walls 7, and an open top 9 at the upper region 11 thereof, the open top exposing a hollow interior or chamber 13 of the element at the top edge 15 of the side walls. The tube sheet 17 comprises a metallic support which supports the ceramic, tubular filter element in an aperture 19, with the walls of the tube sheet about the aperture 19 having an inwardly directed flange 21 upon which an outwardly extending shoulder 23 about the upper region 11 of the ceramic, tubular filter element resets, with a gasket 25 situated between the flange 21 and the shoulder 23. In order to seal the ceramic, tubular filter element 3 to the tube sheet 17, a gasket compression seal is formed by engagement of circular gasket 25 against the top edge 15 of the ceramic, tubular filter element using a compression plate 27 that is bolted to the tube sheet 17 by bolts 29. In another known embodiment, as illustrated in FIG. 2, the filter unit B uses a seal which comprises placement of a metallic cylindrical hollow member 31, as a weight, which may have apertures 33 through a flange 35 thereof, through which positioning posts 37, attached to the surface of the tube sheet 17, may pass. The placement of the weight, on a gasket 39 between the weight and the top edge 15 of the ceramic tubular filter element 3, provides a constant sealing force which is independent of thermal expansion considerations. A primary problem with such a weighted system is that the filter elements typically require a relatively large weight to provide the required sealing force as these filter elements are operated at a significant pressure drop. It is not uncommon to require 20 to 30 pounds weight on each filter element, and this is typically accomplished by using steel weights that are roughly 2 to 3 inches in diameter and 12 to 18 inches long. This greatly affects the high temperature design of the tube sheet which must carry the additional static load and complicates the manifolding of pulse jet cleaning manifolding.
It is an object of the present invention to provide a filter unit wherein a ceramic, tubular filter element is sealed in an opening of a metallic tube sheet by a spring loaded means for use in filtering applications at high temperatures of 700.degree. C. up to in excess of 1000.degree. C.
It is another object of the present invention to provide a filter unit which has a seal between a ceramic, tubular filter element and a metallic tube sheet that can accommodate the differential motion associated with the thermal cycling of component assemblies in high temperature gas filtering operations.